Examples of vortex flowmeters according to a prior art are disclosed in Japanese Utility Model Publication No. SHO-57-25141 and Japanese Patent Publication No. SHO-58-32333, in which a vortex generator is provided in a fluid and the flow rate is measured by detecting the number of Karman's vortices generated on the downstream side of the vortex generator by utilizing ultrasonic waves.
FIG. 6 illustrates the constitution of a vortex flowmeter of a well known type. In FIG. 6, ultrasonic waves which have been generated by the oscillator 1 and transmitted into the fluid by the ultrasonic wave transmitter 2a are caused to propagate in the direction perpendicular to the direction of the flow of fluid as well as along a passage parallel to the plane of the drawing and are detected by the ultrasonic wave receiver 2b. The output signals from the ultrasonic wave receiver 2b are supplied as .alpha. signals to the phase comparator 4 through the phase controller 3. On the other hand, the oscillation signals which have been branched off from the output of the oscillator 1 separately from ultrasonic waves which pass through the fluid are supplied as .beta. signals to the other input terminal of the phase comparator 4 through the phase controller 5.
The phases of the two kinds of signals, .alpha. and .beta., are compared by the phase comparator 4. In this instance, the ultrasonic .alpha. wave signals which have passed through the fluid in which no Karman's vortices are generated possess a certain constant difference in phase relative to the original .beta. oscillation signals.
In FIG. 6, when the fluid the flow rate of which is to be detected flows vertically downward in the pipe line 6 shown in the drawing, regular vortices, or the so-called Karman's vortices are alternately generated in the fluid to the left and right on the downstream side of a well-known vortex generator 7 disposed in the pipe line 6.
In this case, upon propagation in the fluid of the ultrasonic waves passing through the fluid, and when the ultrasonic waves encounter the Karman's vortices generated by the vortex generator 7, the ultrasonic waves will be subjected to phase modulation by the flow rate component in the lateral direction of the Karman's vortices (or the direction parallel to the plane of the drawing and the direction of propagation of the ultrasonic waves). Accordingly, the two kinds of signals, .alpha. and .beta., supplied to the phase comparator 4 exhibit a phase difference which is different from the constant phase difference obtained when ultrasonic waves passing through the fluid do not encounter any Karman's vortex. If this change in the phase difference is detected and output from the output terminal 8a through the filter 8, the number of Karman's vortices which have been generated in proportion to the flow rate may be detected and thus the flow rate of the fluid to be measured may be measured.
It is to be noted, however, that the phase of the ultrasonic waves which are propagated through the fluid will also be caused to change by external phenomena such as a change in temperature of the fluid, etc., apart from any Karman's vortex. It is therefore possible that the difference between the phases of two signals supplied to the phase comparator 4 may deviate from the linear operational range of the phase comparator due to a change in phase caused by such external phenomenon as a temperature change or the like.
In order to prevent this deviation, any change in the phase due to external phenomena is firstly extracted by the filter 8 and the output so processed is supplied to the phase controllers 3 and 5 so that the difference in phases between two ultrasonic waves supplied to the phase comparator 4 may be controlled to ensure that it is included in the linear operational range of the phase comparator 4. As a consequence, even if the phases of ultrasonic waves vary considerably due to such external phenomena as temperature changes, etc., such variation in the phases can be coped with.
With a vortex flowmeter of prior art as explained above, if it is desired to prevent deviation of the difference in the phases of two ultrasonic waves supplied to the phase comparator 4 beyond the linear operational range of the phase comparator 4 due to external phenomena such as changes in fluid temperature and the like, the circuits of the phase controllers 3, 5 would have to be extremely complicated. Moreover, since the S/N ratio of the ultrasonic wave signals for detecting generation of Karman's vortices must be low in a flow having a low Reynold's number, the gain of the amplifier adapted to amplify the frequency and signals of the ultrasonic waves have to be increased, leading to an increase in the consumption of electric power as well.
It is also to be noted that if inexpensive phase controllers 3, 5 are to be employed, such controllers are limited in their ability to accommodate variations in phases and this will mean that the relative vortex flowmeter explained above is unable to work when the extent of variations in the phases due to external phenomena as explained exceeds a given range of tolerance.
In view of the problems pointed out above, the applicant of the present invention proposed a vortex flowmeter as shown in FIG. 7 as the subject of Japanese Published patent application Ser. No. Sho-64-78114, wherein first and second ultrasonic wave transmitters 9a', 10a' which constitute the ultrasonic wave transmitter of this vortex flowmeter are disposed on the wall of the pipe line downstream of the vortex generator and the first and second ultrasonic wave receivers 9b', 10b' which constitute the ultrasonic wave receiver of this vortex flowmeter are disposed on the wall of the pipe line respectively facing the first and second ultrasonic wave transmitters 9a', 10a', the first ultrasonic wave propagation route from the first ultrasonic wave transmitter 9a' to the first ultrasonic wave receiver 9b' and the second ultrasonic wave propagation route from the second ultrasonic wave transmitter 10a' to the second ultrasonic receiver 10b' being disposed parallel to each other but directed in opposite directions, the first and second ultrasonic wave transmitters 9a', 10a' and the first and second ultrasonic wave receivers 9b', 10b' being provided such that the first and second ultrasonic wave propagation routes are caused to simultaneously cross the vortices continuously generated by the vortex generator, and the phases of the respective ultrasonic wave signals output from the first and second ultrasonic wave receivers 9b', 10b' being compared by the phase comparator 11 which allows the vortices continuously generated by the vortex generator to be detected.
However in the arrangement in which the first and second ultrasonic wave generation routes are disposed parallel to each other, if the ultrasonic wave transmitters 9a', 10a' as well as the ultrasonic wave receivers 9b', 10b' are attached to the outer wall surface of the pipe line 6 by means of clamping, refraction of the ultrasonic wave to be propagated is likely to occur at the barrier of the propagation media in the ultrasonic wave propagation route from the ultrasonic wave transmitter 9a' to the ultrasonic wave receiver 9b', as shown in FIG. 8. For this reason, the ultrasonic wave transmitter and the ultrasonic wave receiver have to be so mounted that the ultrasonic wave can be made incident on the propagation media in an inclined fashion, and the incident angle relative to the respective border of the propagation media may thus vary due to temperature changes or the like, making it impossible to reliably transmit and receive ultrasonic waves.
Further in the arrangement where the mounting holes are drilled through the wall of the pipe line so that the ultrasonic wave transmitter and the ultrasonic wave receiver are mounted by insertion in the holes, the tip end portions of the ultrasonic wave transmitters 9a', 10a' and the ultrasonic wave receivers 9b', 10b' are attached to the mounting holes leaving at least partly uneven portions relative to the inner wall surface of the pipe line. Thus creation of uneven portions which are not necessary on the inner wall surface means that the flow rate of the fluid flowing adjacent to the inner wall may vary when passing over the uneven portion of the mounting hole. Accordingly it is preferable that there are no such recessed portions in a vortex flowmeter which may cause disturbance of the fluid as it passes over the inner wall of the pipe line. It is also important that the tip end portions of the respective ultrasonic wave transmitters 9a', 10a' and ultrasonic wave receivers 9b', 10b' are identical in configuration to the inner wall surface of the pipe line 6 if at all possible.
It is therefore the object of the present invention to provide a vortex flowmeter which solves the above-described problems and yet complies with the requirements described above.